Most gases and vapors exhibit well defined optical spectral absorption bands, in which the transmission of optical energy is absorbed. Carbon dioxide (CO2 ) has several absorption bands in the infrared optical range. In particular, a predominant absorption band occurs at a wavelength of 4.26 micrometers. This absorption band is relatively sharply defined within a wavelength range of about 4.26 plus or minus 0.2 micrometers. Infrared radiation transmitted through CO2 gas at a wavelength of 4.26 micrometers is strongly attenuated.
The absorption of infrared radiation in a gas occurs at the atomic and molecular level. In the case of CO2 , the polyatomic molecular structure determines the photon excitation modes and energy exchange rates, and hence, the wavelengths at which optical energy absorption occurs. Because this absorption effect occurs at the molecular level, the absorption of infrared radiation along a given transmission path depends on the number of molecules present. That is, the amount of absorption at 4.26 micrometers is directly proportional to the molecular fraction of CO2 present in a gas mixture. Additionally, because pressure and temperature affect the density of the gas, the absorption is also dependent on the pressure and temperature at which the infrared absorption measurements are made.
This selective optical absorption phenomena has application as a method for determining the presence and amount of CO2 in samples of gas mixtures. For example, a basic CO2 sensing technique in common use employs an infrared radiation source (typically an incandescent lamp) and an infrared detector (typically a semiconductor photodiode) in a closed chamber in which gas samples are introduced for testing. A narrowband interference filter is used as the optical window of the photodiode detector to make it selective only to the 4.26 micrometer absorbing wavelength. For a given optical path length in the test chamber, the photodetector output can be calibrated using gas mixtures having a known CO2 gas concentration to provide a useful instrument for sensing CO2 in a variety of gas mixtures. The basic sensitivity of this arrangement depends on the sharpness of the interference filter so as to minimize the amount of infrared radiation not related to CO2 absorption reaching the detector, the optical path length containing the CO2 molecules, the luminance stability of the infrared radiation source, and the stability of the detection response of the photodiode detector. Other factors that can affect the sensitivity and calibration accuracy of the method include possible turbulent flow in the gas sample passing through the test chamber, aging of the infrared radiation source and detector, and contamination accumulation on the source and detector optical windows.
The effects of turbulent flow can be minimized, and reliable measurements of gas pressure and temperature can be provided, by proper test chamber design. Other sources of error, such as aging effects on the source intensity and detector sensitivity and non-uniform contamination on the optical windows can be minimized by using the 4.26 micrometer wavelength to sense the absorption of CO2 in the presence of the various error-causing factors and also sensing the infrared intensity incident on the detector at a nearby wavelength, such as at 3.9 micrometers, at which no CO2 absorption occurs. Then, by normalizing the CO2 absorption response to the non-absorbing response, the errors associated with all factors other than the molecular absorption of CO2 can be compensated in the sensor output reading. An example of an error-compensated CO2 sensor is described in U.S. Pat. No. 5,646,729, entitled “Single Channel Gas Concentration Measurement Method and Apparatus Using a Short-Resonator Fabry-Perot Interferometer”, issued in 1997 to Koskinen, et al.